1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are well known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air-fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and an improvement of the fuel economy. As an example of the air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533. In conventional air intake side secondary air supply systems, it is usual to control the air-fuel ratio of the mixture to be supplied to the engine toward a single target air-fuel ratio which has been determined previously. However, there are some occasions in driving conditions of the engine where it is desirable to vary the target air-fuel ratio for improving the driveability. Therefore, it is been desirable that the air-fuel ratio of the mixture to be supplied to the engine is controlled to a target air-fuel ratio which is arbitrarily varied according to the driving condition of the engine.
Further, if a fluctuation of the air-fuel ratio of the mixture from the target air-fuel ratio occurs due to a change in the engine load, from a light load to a heavy load for example, it has resulted in a delay of the air-fuel ratio control because of time required for detecting the fluctuation of the air-fuel ratio by means of the O.sub.2 sensor as a change in the oxygen concentration in the exhaust gas. For this reason, there exists a problem of the driveability being deteriorated by the delay of the response of the air-fuel ratio control.